Synchronous reluctance machines known in the art typically comprise stators with poly-phase windings forming a plurality of poles in a manner resembling the stator of an induction motor. The rotor assembly of the synchronous reluctance machine does normally not include electrical windings but has a number of poles in form of magnetically permeable segments. The rotor assembly is formed as an anisotropic structure where each pole of the reluctance machine has a direction of minimum reluctance, a so-called direct axis or d-axis, and a direction of maximum reluctance, a so-called quadrature axis or q-axis. When sinusoidal currents are applied to the poly-phase windings in the stator, an approximately sinusoidal magnetic flux waveform is produced in an air gap formed between the stator poles and an outer contour of the rotor assembly. The rotor will attempt to align its most magnetically permeable direction, the d-axis, to the direction of the peak flux by displacing its d-axis of minimum reluctance until alignment of the magnetic fields in the stator poles and rotor poles is obtained. The alignment process results in rotary motion of the rotor assembly at the same speed as the rotating stator magnetic field, i.e. at synchronous speed. The rotary motion of the rotor generates a torque which can be conveyed to the exterior of the reluctance machine for example by a rotor shaft bonded to the rotor assembly and extending through a central axis thereof.
According to FIG. 1 the rotors 30 are given an anisotropic structure by alternating layers of magnetically permeable segments 3 and magnetically insulating barriers 4. The rotor assembly may comprise a stack of transversally oriented rotor laminations or axially oriented rotor laminations. In the case of transversally oriented laminations shown in FIG. 1 the rotor 30 essentially consists of a stack of thin rotor discs 1 wherein the insulating barriers 4 are created by cutting material in shape of longitudinal slots. It is thereby the air inside the cut-outs that functions as the insulating barrier 4. The disc structures are typically designed to be mechanically self-sustained, the magnetically permeable segments 3 being connected by narrow tangential ribs 16 at the periphery of the discs. Radial ribs 17 close to the middle of the segments are known in order to improve the mechanical strength of the discs. In the case of axially oriented laminations the insulating barriers 4 are made of some solid insulating material and the insulating barriers 4 reach the air gap between the rotor 30 and the stator.
An important parameter of the rotor is a rotor slot pitch which can be defined as the distance between two adjacent insulating layers measured at the air gap between the rotor and the stator. Rotor assemblies have conventionally been designed with an equal rotor slot pitch i.e. all rotor slot pitches being of equal distance, and an equal stator slot pitch, in order to minimize the torque ripple and to provide a reasonable torque. Another conventional practice has been to set the rotor slot pitch equal to the stator slot pitch.
U.S. Pat. No. 5,818,140 discloses a rotor assembly for a synchronous reluctance motor, the structure of the transversely laminated rotor assembly being aimed at minimizing its torque ripple. The disclosed rotor design and the accompanying design formula require an equal rotor slot pitch and an equal stator slot pitch around the respective perimeters of these components.
U.S. Pat. No. 6,239,526 discloses a rotor assembly for a synchronous reluctance motor, wherein the insulating barriers of the rotor are slanted toward the q-axis. The rotor slot pitch has thereby been rendered non-equal across both q-axis and d-axis, the aim being to minimize the torque ripple by dimensioning the insulating barriers such that while one end faces a centre of a slot of the stator, the other end faces a centre of a tooth of the stator. This disclosure assumes that the rotor slot pitch between the q-axis and the d-axis of each pole is equal to the stator slot pitch.
However, despite the measures taken to decrease the torque ripple, rotors designed according to the conventional design principles still exhibit a high torque ripple.